/Engine/Source/Runtime/Core/Public/Algo/Partition.h
Rearranges the elements so that all the elements for which Predicate returns true precede all those for which it returns false. (not stable)
index of the first element in the second group
namespace Algo { template<class T, typename IndexType, typename UnaryPredicate> IndexType Algo::Partition ( T * Elements, const IndexType Num, UnaryPredicate Predicate ) }